Typically, a case for a tube of lipstick may comprise a cup into which a lower portion of the stick is inserted. The case comprises an inner body fitted into an outer body, and control elements for rotating the inner body relative to the outer body about a main axis. The cup is guided in translation along the main axis relative to the outer and inner body by means of a helical-type connection for example. More specifically, the inner body comprises for example vertical grooves, typically two, extending along the main axis, and the outer body comprises helical grooves, again typically two, the cup comprising tabs designed to engage with both the vertical grooves and the helical grooves. Thus, a rotation between the inner body and the outer body causes translational movement of the cup. An external sleeve, fixed to the outer body, may be added in order to provide, for example, a tidy finish or decorative elements.
A problem frequently encountered in lipstick cases concerns the minimum torque that must be applied to the inner body to rotate it relative to the outer body. If the torque is too low, the inner body could rotate within the outer body without any action by the user, simply due to gravity or impacts during transport. The stick of product would then protrude from the case unexpectedly and could be damaged. On the other hand, if the torque is too high, the user has to apply significant force to the inner body to cause it to rotate, which can be perceived as an annoyance by the user.
It is therefore necessary to provide a torque to be applied to the inner body for its rotation that ensures easy manipulation by the user, while limiting undesired rotation.
Several solutions for this have been investigated. For example, a substantially tight fit can be implemented between at least a portion of the inner body and of the outer body, with a lubricant enabling rotation between the inner body and the outer body. However, in addition to increasing costs, the use of a lubricant can result in odors unpleasant to the user. Leakage may also occur, both externally which is distasteful to the user and internally where it contaminates the product.
This is why a flexible lip has been developed, provided for example by the inner body and bearing against the outer body, exerting pressure against it. A gap is formed between the inner body and the outer body, and the lip ensures physical contact between the inner body and outer body. The lip allows applying minimal frictional torque to obtain rotation, while bridging the gap between the inner body and the outer body.
An example of such a lip is given in EP 0 491 579. The lip is presented therein in the form of an annular ring of triangular cross-section, formed on the inner body. The ring fits into a similarly annular groove formed in the outer body, and presses against the bottom of this groove.
Patent EP 0 943 262 gives a similar example of such a lip, in the form of a circular contact rib provided on the inner body and forming a line of contact with the outer body.
However, although they allow controlling the torque between the inner body and outer body, the lips proposed by the prior art have several disadvantages.
In particular, the lips proposed by the prior art extend along the perimeter of the inner body, and as a result provide a contact surface between the inner body and outer body that is limited by the size of the perimeter of the inner body. The minimum torque is therefore also limited.
In addition, the dimensions of the lips of cases from the same manufacturing line are not necessarily stable. For example, in the case where the inner body and the lip are obtained by molding, dimensional variation between two inner bodies is inevitable even if they are from the same mold. However, the transverse dimension of the lip must be sufficient to be able to absorb dimensional differences, but must also ensure the desired torque between the inner body and outer body. Thus, for example, the greater the transverse dimension of the lip, the better the absorption of dimensional differences but the greater the rotational torque required between the inner body and outer body. This causes differences in rotational torque between cases originating from the same manufacturing process, and therefore differences in performance that may be perceived by the user as differences in quality.
The invention aims to propose a solution to the abovementioned problems of the prior art concerning lipstick cases.
More particularly, the invention proposes a lip that allows imposing torque between the inner body and the casing, and that has a novel form easily adaptable to the use required, without significantly increasing manufacturing costs.